Element having antenna array structure

ABSTRACT

An element includes a coupling line in which a first conductor layer, a dielectric layer, and a second conductor layer are stacked in this order, and which is connected to the second conductor layer in order to mutually synchronize a plurality of antennas at a frequency of a terahertz wave; and a bias line connecting a power supply for supplying a bias signal to a semiconductor layer and the second conductor layer. A wiring layer in which the coupling line is formed and a wiring layer in which the bias line is formed are different layers. The bias line is disposed in a layer between the first conductor layer and the second conductor layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of International Patent ApplicationNo. PCT/JP2020/028567, filed Jul. 22, 2020, which claims the benefit ofJapanese Patent Application No. 2019-152828, filed Aug. 23, 2019, bothof which are hereby incorporated by reference herein in their entirety.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to an element.

Background Art

As a current injection type light source that produces anelectromagnetic wave in the frequency range of at least 30 GHz and notmore than 30 THz (hereinafter referred to as “terahertz wave”), there isknown an oscillator in which a semiconductor element having anelectromagnetic wave gain for the terahertz wave and a resonator areintegrated. Among oscillators of such kind, an oscillator in which aresonant tunneling diode (RTD) and an antenna are integrated is expectedas an element that operates at room temperature in a frequency rangearound 1 THz.

PTL 1 discloses an antenna array for the terahertz wave in which aplurality of oscillators, in each of which an RTD and an antenna areintegrated, are arranged on the same substrate.

In the antenna array disclosed in PTL 1, an increase in antenna gain canbe expected by increasing the number of antennas and synchronizing theantennas. On the other hand, coupling lines for coupling the adjacentantennas for synchronizing the oscillators and bias lines for supplyinga bias signal to the RTDs are required. Accordingly, as the number ofantennas increases, the risk of electrical and mechanical interferencebetween the coupling line and the bias line for each antenna increases.Therefore, since there is a limit to the number of antennas that can bearranged, resulting in a limit to the effect of enhancing the power andgain by the antenna array, this makes it impossible to efficientlyproduce and detect a terahertz wave.

SUMMARY OF THE INVENTION

In view of the above-described problems, a purpose of the presentdisclosure of technology is to provide efficient generation or detectionof a terahertz wave in an element having an antenna array structure.

A first aspect of the present disclosure is: an element including: anantenna array in which a plurality of antennas are arranged, with eachof the antennas including a first conductor layer, a semiconductor layerthat is electrically connected to the first conductor layer and producesor detects a terahertz wave, a second conductor layer that iselectrically connected to the semiconductor layer and faces the firstconductor layer via the semiconductor layer, and a dielectric layer thatis located between the first conductor layer and the second conductorlayer; a coupling line that is connected to the second conductor layerconfigured to make mutual synchronization between the plurality ofantennas at a frequency of the terahertz wave; and a bias line thatconnects a power source for supplying a bias signal to the semiconductorlayer and the second conductor layer, wherein a wiring layer in whichthe coupling line is formed and a wiring layer in which the bias line isformed are different layers, and the bias line is disposed in a layerbetween the first conductor layer and the second conductor layer.

A second aspect of the present disclosure is: an element including: anantenna array in which a plurality of antennas are arranged, with eachof the antennas including a first conductor layer, a semiconductor layerthat is electrically connected to the first conductor layer and producesor detects a terahertz wave, a second conductor layer that iselectrically connected to the semiconductor layer and faces the firstconductor layer via the semiconductor layer, and a dielectric layer thatis located between the first conductor layer and the second conductorlayer; a coupling line that is connected to the second conductor layerconfigured to make mutual synchronization between the plurality ofantennas at a frequency of the terahertz wave; and a bias line thatconnects a power source for supplying a bias signal to the semiconductorlayer and the second conductor layer, wherein a wiring layer in whichthe coupling line is formed and a wiring layer in which the bias line isformed are different layers, the element further comprising a thirdconductor layer and a fourth conductor layer, wherein the coupling lineis formed of the third conductor layer and the first conductor layer,the bias line is formed of the fourth conductor layer, the thirdconductor layer and the fourth conductor layer are arranged in differentlayers, and the first conductor layer, the third conductor layer, andthe fourth conductor layer are stacked in this order.

A third aspect of the present disclosure is: an element including: anantenna array in which a plurality of antennas are arranged, with eachof the antennas including a first conductor layer, a semiconductor layerthat is electrically connected to the first conductor layer and producesor detects a terahertz wave, a second conductor layer that iselectrically connected to the semiconductor layer and faces the firstconductor layer via the semiconductor layer, and a dielectric layer thatis located between the first conductor layer and the second conductorlayer; a coupling line that is connected to the second conductor layerconfigured to make mutual synchronization between the plurality ofantennas at a frequency of the terahertz wave; and a bias line thatconnects a power source for supplying a bias signal to the semiconductorlayer and the second conductor layer, wherein a wiring layer in whichthe coupling line is formed and a wiring layer in which the bias line isformed are different layers, and adjacent antennas in the antenna arrayare connected to a common bias line disposed between the adjacentantennas.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a diagram illustrating a semiconductor element according to afirst embodiment;

FIG. 1B is a diagram illustrating the semiconductor element according tothe first embodiment;

FIG. 1C is a diagram illustrating the semiconductor element according tothe first embodiment;

FIG. 2 is a top view of the semiconductor element according to the firstembodiment;

FIG. 3 is a graph of the relationship between dielectric layer thicknessand conductor loss;

FIG. 4A is a diagram illustrating a semiconductor element according to asecond embodiment;

FIG. 4B is a diagram illustrating the semiconductor element according tothe second embodiment;

FIG. 4C is a diagram illustrating the semiconductor element according tothe second embodiment;

FIG. 5A is a diagram illustrating a semiconductor element according to athird embodiment;

FIG. 5B is a diagram illustrating the semiconductor element according tothe third embodiment;

FIG. 5C is a diagram illustrating the semiconductor element according tothe third embodiment;

FIG. 6A is a diagram illustrating a semiconductor element according to afourth embodiment;

FIG. 6B is a diagram illustrating the semiconductor element according tothe fourth embodiment;

FIG. 6C is a diagram illustrating the semiconductor element according tothe fourth embodiment;

FIG. 7 is a diagram illustrating a semiconductor element according tothe fourth embodiment;

FIGS. 8A to 8C are diagrams illustrating the semiconductor elementaccording to the fourth embodiment;

FIGS. 9A and 9B are diagrams illustrating an oscillating elementaccording to a fifth embodiment;

FIG. 10 is a diagram illustrating an oscillating element according to asecond example;

FIGS. 11A and 11B are diagrams illustrating the oscillating elementaccording to the second example;

FIG. 12 is a graph of the relationship between third conductor layer andoscillation frequency;

FIGS. 13A to 13C are graphs showing the influences of the oscillatingoutput of the oscillating element;

FIG. 14 is a graph showing the oscillation outputs of the oscillatingelement according to the second example and a single antenna;

FIG. 15 is a flowchart illustrating a method for manufacturing theoscillating element according to the second example;

FIGS. 16A to 16H are diagrams illustrating steps of processing ofmanufacturing the oscillating element according to the second example;and

FIG. 17A is a diagram illustrating an oscillating element according to athird example; and FIG. 17B is a diagram illustrating an oscillatingelement according to a fourth example.

DESCRIPTION OF THE EMBODIMENTS

<First Embodiment>: A semiconductor element 100 according to a firstembodiment will be described with reference to FIGS. 1A to 2C. Thesemiconductor element 100 is a semiconductor element that produces anoscillating terahertz wave at a frequency of f_(THz) or detects aterahertz wave. FIG. 1A is a perspective view illustrating theappearance of the semiconductor element 100, FIG. 1B is a sectionalview, taken along line A-A′, of the semiconductor element 100, and FIG.1C is a sectional view, taken along line B-B′, of the semiconductorelement 100. FIG. 2 is a top view of the semiconductor element 100 asviewed from the stacking direction of the semiconductor element (fromabove). Note that, in the following description, an example of thesemiconductor element 100 being used as an oscillator will be described.Here, the terahertz wave is an electromagnetic wave in the frequencyrange of at least 30 GHz and not more than 30 THz. Further, the lengthof each of the components of the semiconductor element 100, such as asubstrate 113, a dielectric layer 104, and a semiconductor layer 115, inthe stacking direction of the components is referred to as “thickness”or “height”. Further, the direction in which the dielectric layer 104and the semiconductor layer 115 are located with respect to thesubstrate 113 is referred to as “upper”.

The semiconductor element 100 is provided with a plurality of antennas.In the present embodiment, the semiconductor element 100 includes anantenna array in which nine antennas 100 a, 100 b, 100 c, 100 d, 100 e,100 f, 100 g, 100 h, and 100 i are arranged in a 3×3 matrix. The antenna100 a also serves as a resonator that resonates with the terahertz waveand a radiator that transmits or receives the terahertz wave. Theantenna 100 a has a semiconductor layer 115 a for producing anoscillating electromagnetic wave of terahertz wave or detecting anelectromagnetic wave inside. Each of the other eight antennas 100 b to100 i has the same configuration as the antenna 100 a. Further, theantennas can be arranged at a pitch not more than the wavelength of theterahertz wave to be detected or produced or a pitch of an integralmultiple of the wavelength.

Hereinafter, the configuration of the antenna 100 a will be described indetail, and detailed description of the components of the other antennas100 b to 100 i that are the same as or similar to those of the antenna100 a will be omitted. Further, throughout the description, to the endof the reference numeral for each component of the antennas 100 a to 100i, the alphabet used for the corresponding antenna is appended. Forexample, among second conductor layers 103, the corresponding componentof the antenna 100 a is represented as a second conductor layer 103 a.

(Antennas): The antenna 100 a has a configuration in which thedielectric layer 104 is interposed between two conductor layers (wiringlayers): a first conductor layer 106 and the second conductor layer 103a. Such a configuration is called a microstrip type antenna using amicrostrip line having a finite length or the like. In the presentembodiment, an example using a patch antenna which is a microstrip typeresonator will be described.

The second conductor layer 103 a is a patch conductor for the antenna100 a, disposed so as to face the first conductor layer 106 via thedielectric layer 104 (semiconductor layer 115 a). The second conductorlayer 103 a is electrically connected to the semiconductor layer 115 a.The antenna 100 a is configured to operate as a resonator in which thewidth of the second conductor layer 103 a in the A-A′ direction(resonance direction) is λ_(THz)/2. The first conductor layer 106 is agrounding conductor that is electrically grounded. Note that λ_(THz) isthe effective wavelength of the terahertz wave with which the antenna100 a resonates in the dielectric layer 104, and it is represented asλ_(THz)=λ₀×ε_(r) ^(−1/2), where λ₀ is the wavelength of the terahertzwave in vacuum, and ε_(r) is the effective relative dielectric constantof the dielectric layer 104.

The semiconductor layer 115 a includes an active layer 101 a composed ofa semiconductor layer having an electromagnetic wave gain ornon-linearity for the terahertz wave. As a typical semiconductor layerhaving an electromagnetic wave gain in a frequency band of terahertzwaves, there is known a resonant tunneling diode (RTD). In the presentembodiment, an example in which an RTD is used as the active layer 101 awill be described. Hereinafter, the active layer 101 a is referred to asthe RTD 101 a.

The RTD 101 a includes a resonance tunnel structure layer including aplurality of tunnel barrier layers, in which a quantum well layer isprovided between the plurality of tunnel barriers, and has a multiplequantum well structure in which a terahertz wave is produced byintersubband transition of carriers. The RTD 101 a has anelectromagnetic wave gain in a frequency range of terahertz waves basedon the photon-assisted tunneling phenomenon in the differential negativeresistance region of the current-voltage characteristic, andself-oscillates in the differential negative resistance region.

The antenna 100 a is an active antenna in which the semiconductor layer115 a including the RTD 101 a and a patch antenna are integrated. Thefrequency f_(THz) of the terahertz waves emitted from the antenna 100 aalone is determined by the resonance frequency of an all-parallelresonant circuit in which the patch antenna and the reactance of thesemiconductor layer 115 a are combined. Specifically, from theequivalent circuit of the oscillator described in NPL 1, a frequency isdetermined, as the oscillation frequency f_(THz), that satisfies anamplitude condition represented by Equation 1 and a phase conditionrepresented by Equation 2 are satisfied for the resonance circuit inwhich the admittances of the RTD and the antenna (Y_(RTD) and Y_(aa))are combined.

Re[Y _(RTD)]+Re[Y _(aa)]≤0   (Equation 1)

Im[Y _(RTD)]+Im[Y _(aa)]=0   (Equation 2)

Here, Y_(RTD) is the admittance of the semiconductor layer 115 a, Re isa real part, and Im is an imaginary part. The semiconductor layer 115 aincludes the RTD 101 a which is a negative resistance element as anactive layer, and thus Re[Y_(RTD)] has a negative value. Further, Y_(aa)indicates the admittance of the entire structure of the patch antenna100 a in view of the semiconductor layer 115 a.

Note that, as the active layer 101 a, a quantum cascade laser (QCL)structure having a semiconductor multilayer structure of several hundredto several thousand layers may be used. In this case, the semiconductorlayer 115 a is a semiconductor layer including a QCL structure. Further,as the active layer 101 a, a negative resistance element such as a Gunndiode or an IMPATT diode often used in the millimeter wave band may beused. Further, as the active layer 101 a, a high frequency element suchas a transistor with one terminal terminated may be used, and suitabletransistors include a heterojunction bipolar transistor (HBT), acompound semiconductor layer FET, and a high electron mobilitytransistor (HEMT). Further, as the active layer 101 a, the differentialnegative resistance of a Josephson element using superconductor layersmay be used.

The dielectric layer 104 is composed of two layers: a first dielectriclayer 1041 and a second dielectric layer 1042. In a microstrip typeresonator such as a patch antenna, the large thickness of the dielectriclayer 104 reduces conductor loss and improves radiation efficiency. Thedielectric layer 104 is required to have a possible thick film(typically, 3 μm or more), a low loss and a low dielectric constant in aterahertz band, and a high ease of microfabrication (flattening andetching). Here, the radiation efficiency increases as the thickness ofthe dielectric layer 104 increases, but multi-mode resonance may occurif the thickness is too large. Therefore, it is preferable to design thethickness of the dielectric layer 104 in a range of up to 1/10 of theoscillation wavelength as the upper limit. On the other hand, since aminiaturized, high current density diode is necessary in order toincrease the frequency and output of the oscillator, the dielectriclayer 104 is required to prevent current leakage and address migrationin order to serve as an insulating structure for the diode. In thepresent embodiment, in order to achieve the above two purposes, twotypes of dielectric layers having different materials are used for thefirst dielectric layer 1041 and the second dielectric layer 1042.

Specific examples of the material preferably used for the firstdielectric layer 1041 include organic dielectric materials such as BCB(benzocyclobutene, available from Dow Chemical Co., Ltd., ε_(r1)=2),polytetrafluoroethylene, or polyimide. Here, ε_(r1) is the relativedielectric constant of the first dielectric layer 1041. Further, aninorganic dielectric material such as a TEOS oxide film or spin-onglass, which can form a relatively thick film and has a low dielectricconstant, may be used for the first dielectric layer 1041.

Further, the second dielectric layer 1042 is required to have aninsulating property (a property of acting as an insulator andhigh-resistance resistor that does not conduct electricity as a DCvoltage is applied), a barrier property (a property of preventing ametal material used for an electrode from diffusing), and aprocessability (a property of being processed with submicron accuracy).Specific examples of the material preferably used for satisfying suchproperties include inorganic insulator materials such as silicon oxide(ε_(r2)=4), silicon nitride (ε_(r2)=7), aluminum oxide, or aluminumnitride. Here, ε_(r2) is the relative dielectric constant of the seconddielectric layer 1042.

Here, in the case where the dielectric layer 104 has a two-layerstructure as in the present embodiment, the relative dielectric constantε_(r) of the dielectric layer 104 is an effective relative dielectricconstant determined from the thickness and the relative dielectricconstant ε_(r1) of the first dielectric layer 1041 and the thickness andthe relative dielectric constant ε_(r2) of the second dielectric layer1042. Further, from the viewpoint of impedance matching between antennaand space, it is preferable that the difference in dielectric constantbetween antenna and air is small. Thus, the first dielectric layer 1041is made of a material different from that of the second dielectric layer1042, and the material preferably has a relative dielectric constantlower than that of the second dielectric layer 1042 (ε_(r1)<ε_(r2)).Note that, in the semiconductor element 100, the dielectric layer 104does not have to have a two-layer structure, and may have a structurecomposed of only one layer made of the above-mentioned materials.

The semiconductor layer 115 a is disposed on the first conductor layer106 formed on the substrate 113. The semiconductor layer 115 a and thefirst conductor layer 106 are electrically connected to each other. Notethat, in order to reduce the ohmic loss, it is preferable that thesemiconductor layer 115 a and the first conductor layer 106 areconnected with low resistance. An electrode 116 a is disposed on theside opposite to the side where the first conductor layer 106 isdisposed with respect to the semiconductor layer 115 a, and theelectrode 116 a and the semiconductor layer 115 a are electricallyconnected to each other. The semiconductor layer 115 a and the electrode116 a are embedded in the second dielectric layer 1042 and aresurrounded by the second dielectric layer 1042.

If the electrode 116 a is a conductor that makes ohmic contact with thesemiconductor layer 115 a, it is suitable for reducing the ohmic lossand RC delay due to the series resistance. For the electrode 116 a beingused as an ohmic electrode, examples of the material preferably usedinclude Ti/Pd/Au, Ti/Pt/Au, AuGe/Ni/Au, TiW, Mo, ErAs, and the like.Further, if the region of the semiconductor layer 115 a in contact withthe electrode 116 a is a semiconductor doped with high concentration ofimpurities, the contact resistance becomes lower, which is suitable forhigh output and high frequency. Since the absolute value of the negativeresistance indicating the magnitude of the gain of the RTD 101 a used inthe terahertz wave band is on the order of about 1 to 100Ω, it ispreferable to suppress the loss of the electromagnetic wave to 1% orless. Thus, the contact resistance of the ohmic electrode is preferablydesigned to have 1Ω or less as a guide. Further, in order to operate inthe terahertz wave band, the width of the semiconductor layer 115 a(almost equal to the electrode 116 a) is typically about 0.1 to 5 μm.Accordingly, it is preferable to set the resistivity of the contactinterface between the semiconductor layer 115 a and the electrode 116 ato 10Ω·m² or less to suppress the contact resistance to the range of0.001 to several Ω.

Further, it is conceivable to use a metal that makes Schottky contactwith the electrode 116 a, instead of ohmic contact. In this case, thecontact interface between the electrode 116 a and the semiconductorlayer 115 a exhibits rectification, and the antenna 100 a has a suitablestructure as a terahertz wave detector. Hereinafter, in the presentembodiment, a configuration using an ohmic electrode as the electrode116 a will be described.

Inside the antenna 100 a arranged above and below the RTD 101 a, asillustrated in FIG. 1B, the substrate 113, the first conductor layer106, the semiconductor layer 115 a, the electrodes 116 a, a conductor117 a, and the second conductor layer 103 a are stacked in this order.

The conductor 117 a is formed inside the dielectric layer 104. Thesecond conductor layer 103 a and the electrode 116 a are electricallyconnected to each other via the conductor 117 a. Here, if the width ofthe conductor 117 a is too large, the radiation efficiency is reduceddue to the deterioration of the resonance characteristic of the patchantenna 100 a and the increase of the parasitic capacitance. Thus, thewidth of the conductor 117 a preferably has a dimension such that itdoes not interfere with the resonant electric field, and is typically1/10 or less of the effective wavelength λ of the terahertz wave havingthe oscillation frequency of f_(THz) standing in the antenna 100 a.Further, the width of the conductor 117 a may be reduced to the extentthat it does not increase the series resistance, and as a guide, it canbe reduced to about twice the skin depth. Considering that the seriesresistance is reduced to the extent that it does not exceed 1Ω, thewidth of the conductor 117 a is typically in the range of at least 0.1μm and not more than 20 μm as a guide.

The second conductor layer 103 a is electrically connected to a line 108a 1 and a line 108 a 2 via a conductor 107 a 1 and a conductor 107 a 2.Further, the lines 108 a 1 and 108 a 2 are lead wires electricallyconnected to a bias circuit 120 via a bias line 111 which is a commonwire formed in the chip. The lines 108 are each drawn from the antenna.The bias circuit 120 is a power source for supplying a bias signal tothe RTD 101 a of the antenna 100 a. Accordingly, the bias signal issupplied to the semiconductor layer 115 of each antenna by theconnection between the bias line 111 and the lines 108 which are leadwires drawn from the adjacent antennas. Since the bias line 111 iscommon, the variation in operating voltage between the antennas can bereduced, so that synchronization is stabilized even for an increasednumber of antennas in the array. In addition, the structure around eachantenna can be made symmetrical, resulting in no deformation of theradiation pattern.

The conductors 107 a 1 and 107 a 2 are connection portions forelectrically and mechanically connecting the lines 108 a 1 and 108 a 2to the second conductor layer 103 a. A structure that electricallyconnects the upper and lower layers, such as the conductor 117 a, theconductor 107 a 1, or the conductor 107 a 2, is called a via. The firstconductor layer 106 and the second conductor layer 103 a not only serveas members constituting the patch antenna, but also serve as electrodesfor injecting a current into the RTD 101 a by being connected to thevias for them. For the conductor 117 a and the conductors 107 a 1 and107 a 2 which are vias, a material having a resistivity of 1×10⁻⁶Ω·m orless is preferably used. Specifically, examples of the materialpreferably used include metals, such as Ag, Au, Cu, W, Ni, Cr, Ti, Al,AuIn alloy, and TiN, and their compounds.

The width of each of the conductors 107 a 1 and 107 a 2 is smaller thanthe width of the second conductor layer 103 a. The width as referred toherein is a width in the electromagnetic wave resonance direction (=A-A′direction) in the antenna 100 a. Further, the width of a part(connection part) of the line 108 a 1 (line 108 a 2) connected to theconductor 107 a 1 (conductor 107 a 2) is smaller (narrower) than thewidth of the second conductor layer 103 a (antenna 100 a). Further,these widths are each preferably 1/10 or less (λ/10 or less) of theeffective wavelength λ of the terahertz wave having the oscillationfrequency f_(THz) standing in the antenna 100 a. This is because it ispreferable to arrange the conductors 107 a 1, 107 a 2 and the lines 108a 1, 108 a 2 in dimensions and positions such that they do not interferewith the resonance electric field in the antenna 100 a in order toimprove the radiation efficiency.

Further, it is preferable that the positions of the conductors 107 a 1and 107 a 2 are arranged in the electric field nodes of the terahertzwave having the oscillation frequency f_(THz) standing in the antenna100 a. In this case, the conductors 107 a 1, 107 a 2 and the lines 108 a1, 108 a 2 have a configuration in which the impedance is sufficientlyhigher than the absolute value of the differential negative resistanceof the RTD 101 a in a frequency band around the oscillation frequencyf_(THz). In other words, the lines 108 a 1 and 108 a 2 are connected toantennas other than the antenna 100 a so as to have high impedance forthe RTD at the oscillation frequency f_(THz). In this case, the otherantennas and the antenna 100 a are isolated (separated) in a path viathe bias line 111 at the frequency f_(THz). As a result, a current withthe oscillation frequency f_(THz) induced in each antenna via the biasline 111 and the bias circuit 120 does not affect the adjacent antenna.Further, the interference between the electric field having theoscillation frequency f_(THz) standing in the antenna 100 a and thesefeeding members is suppressed. The same applies to the other antennas100 b to 100 i in the semiconductor element 100 as well as the antenna100 a.

The bias line 111 is a common bias wire (wiring layer) for the antennas100 a to 100 i. The each of antennas 100 a to 100 i is connected to thebias line 111 via one of the lines 108 a 1, 108 a 2 to 108 i 1, 108 i 2connected to the one of the antennas 100 a to 100 i. In FIGS. 1B and 1C,the bias line 111 is illustrated in which wires in the A-A′ direction(resonance direction) are labeled with 111 x 1 to 111 x 4, and wires inthe B-B′ direction are labeled with 111 y 1 to 111 y 4. Note that, inthis description, the entire bias common wire for the semiconductorelement 100 is referred to as the bias line 111.

((Bias Circuit)): The bias circuit 120 is a power supply disposedoutside the chip in order to supply a bias signal to the RTDs 101 a to101 i. The bias circuit 120 includes a shunt resistor 121 which isconnected in parallel with each of the RTDs 101 a to 101 i, a wire 122,a power supply 123, and a capacitor 124 (a capacitor connected inparallel with the shunt resistor 121).

The wire 122 is illustrated as an inductance in FIG. 1A because italways has a parasitic inductance component. The power supply 123supplies a current required to drive each of the RTDs 101 a to 101 i,and adjusts a bias voltage applied to each of the RTDs 101 a to 101 i.The bias voltage is typically selected from voltages in the differentialnegative resistance region of the RTD used for the RTDs 101 a to 101 i.The bias circuit 120 is connected to the bias line 111, which is anin-chip wire. For the antenna 100 a, the bias voltage from the biascircuit 120 is supplied to the RTD 101 a in the antenna 100 a via theline 108 a 1 and the line 108 a 2. The same applies to the otherantennas 100 b to 100 i as well as the antenna 100 a.

The shunt resistor 121 and the capacitor 124 have a role of suppressingparasitic oscillation with a resonance frequency of relatively lowfrequency (typically, a frequency band from DC (direct current) to 10GHz) caused by the bias circuit 120. The shunt resistor 121 is selectedto have a value equal to or slightly smaller than the absolute value ofthe combined differential negative resistance of the RTDs 101 a to 101 iwhich are connected in parallel. Similar to the shunt resistor 121, thecapacitor 124 is also set to have a value such that the impedance of theelement is equal to or slightly lower than the absolute value of thecombined differential negative resistance of the RTDs 101 a to 101 iwhich are connected in parallel. In other words, the bias circuit 120 isset by such a shunt structure to have an impedance lower than theabsolute value of a combined negative resistance corresponding to thegain in a frequency band from DC to 10 GHz. Generally, the capacitor 124preferably has a larger capacitance as long as it is within theabove-mentioned range, and in the example of the present embodiment, acapacitance of about several tens of pF. The capacitor 124 is adecoupling capacitor, and for example, may make use of a MIM(Metal-insulator-Metal) structure in which the antenna 100 a and thesubstrate are integrated.

(Antenna Array): The semiconductor element 100 is an antenna arrayincluding the nine antennas 100 a, 100 b, 100 c, 100 d, 100 e, 100 f,100 g, 100 h, and 100 i, which are arranged in a 3×3 matrix. Each of theantennas 100 a to 100 i alone produces an oscillating terahertz wave ata frequency of f_(THz). The adjacent antennas are mutually coupled by acoupling line 109, so that they are in mutual injection locking (mutualsynchronization) with the oscillation frequency f_(THz) of the terahertzwave.

Here, the mutual injection locking means that a plurality ofself-excited oscillators oscillate in entrainment synchronization bytheir interaction. For example, the antenna 100 a and the antenna 100 bare mutually coupled by a coupling line 109 ab, and the antenna 100 aand the antenna 100 d are mutually coupled by a coupling line 109 ad.The same applies to the other adjacent antennas. Note that “mutuallycoupled” refers to a phenomenon in which a current induced in oneantenna acts on another adjacent antenna to change the transmission andreception characteristics of each other. By synchronizing the mutuallycoupled antennas in the same phase or the opposite phase, the mutualinjection locking phenomenon causes the electromagnetic fields to bestrengthened or weakened between the antennas. This makes it possible toadjust the increase and decrease of the antenna gain. Note that, in thisdescription, when the entire coupling line connecting the antennas ofthe semiconductor element 100 is represented, it is referred to as thecoupling line 109. Further, each coupling line connecting the antennasincluded in the coupling line 109 is referred to using the alphabet forthe corresponding antenna. For example, the coupling line connecting theantenna 100 a and the antenna 100 b is referred to as the coupling line109 ab.

The oscillation conditions of the semiconductor element 100 aredetermined by the conditions of mutual injection locking in aconfiguration in which two or more individual RTD oscillators arecoupled, which is disclosed in J. Appl. Phys., Vol. 103, 124514 (2008)(NPL 2). Specifically, now consider the oscillation conditions of theantenna array in which the antenna 100 a and the antenna 100 b arecoupled by the coupling line 109 ab. In this case, two oscillation modesare possible: a positive phase mutual injection locking and a negativephase mutual injection locking. The oscillation conditions for theoscillation mode (even mode) of the positive phase mutual injectionlocking are represented by Equations 4 and 5, and the oscillationconditions for the oscillation mode (odd mode) of the negative phasemutual injection locking are represented by Equations 6 and 7.

Positive phase (even mode): frequency f=f_(even)

Y _(even) =Yaa+Yab+Y _(RTD)

Re(Y _(even))≤0   (Equation 4)

Im(Y _(even))=0

Negative phase (odd mode): frequency f=f_(odd)

Y _(odd) =Yaa+Yab+Y _(RTD)   (Equation 5)

Re(Y _(odd))≤0 (Equation 6)

Im(Y _(odd))=0 (Equation 7)

Here, Y_(ab) is a mutual admittance between the antenna 100 a and theantenna 100 b. Y_(ab) is proportional to a coupling constantrepresenting the strength of the coupling between the antennas, andideally, the real part of −Y_(ab) is large and the imaginary part iszero. The semiconductor element 100 of the present embodiment is coupledunder the condition for the positive phase mutual injection locking, andthe oscillation frequency f_(THz) is almost equal to f_(even).Similarly, for the other antennas, the adjacent antennas are mutuallycoupled by the coupling line 109 so that the above-mentioned conditionsfor the positive phase mutual injection locking are satisfied.

The coupling line 109 is a microstrip line in which the dielectric layer104 is interposed between the third conductor layer 110 and the firstconductor layer 106. For example, as illustrated in FIG. 1B, thecoupling line 109 ab has a structure in which the dielectric layer 104is interposed between the third conductor layer 110 ab and the firstconductor layer 106. Similarly, the dielectric layer 104 is interposedbetween the first conductor layer 106, and the third conductor layer 110bc in the coupling line 109 bc; the third conductor layer 110 ad in thecoupling line 109 ad; and the third conductor layer 110 cf in thecoupling line 109 cf.

In the semiconductor element 100, the adjacent antennas are coupled byDC coupling. The third conductor layer 110 ab, which is the upperconductor layer of the coupling line 109 ab for coupling the antenna 100a and the antenna 100 b, is directly connected to the second conductorlayers 103 a and 103 b. In the semiconductor element 100, the thirdconductor layer 110 ab and the second conductor layers 103 a and 103 bare formed in the same layer. Similarly, the third conductor layer 110ae, which is the upper conductor layer of the coupling line 109 ae forcoupling the antenna 100 a and the antenna 100 e, is directly connectedto the second conductor layers 103 a and 103 e. The third conductorlayer 110 ae and the second conductor layers 103 a and 103 e are formedin the same layer.

With this structure, the antenna 100 b and the antenna 100 e aremutually coupled for the antenna 100 a, and operate in mutualsynchronization with the frequency f_(THz) of the oscillating terahertzwave. In the antenna array synchronized by such DC coupling, theadjacent antennas can be synchronized by a strong coupling, so that itis possible to facilitate entrainment synchronization operation, whichis robust to variations in frequency and phase of the antennas.

Note that, in the semiconductor element 100, the coupling line 109 andthe bias line 111 are arranged in different layers. For example, asillustrated in FIG. 1B, the third conductor layer 110 ab forming thecoupling line 109 ab for coupling the antenna 100 a and the antenna 100b and the fourth conductor layer 111 x 2 forming the bias line 111 arearranged in different layers. Further, the third conductor layer 110 adforming the coupling line 109 ad for coupling the antenna 100 a and theantenna 100 d and the fourth conductor layer 111 x 1 forming the biasline 111 are arranged in different layers. In other words, the wiringlayers in which parts of the coupling line 109 extending in the in-planedirection (direction perpendicular to the stacking direction) of thesubstrate 113 are formed and the wiring layer in which a part of thebias line 111 extending in the in-plane direction of the substrate 113is formed are arranged in different layers. Here, the wiring layers inwhich the parts of the coupling line 109 extending in the in-planedirection are formed are the third conductor layer 110 and the firstconductor layer 106. On the other hand, the wiring layer in which thepart of the bias line 111 extending in the in-plane direction is formedis the fourth conductor layer 111. Further, when a virtual plane whichis the plane in which the first conductor layer 106 extends is taken,the distance between the coupling line 109 and the virtual plane isdifferent from the distance between the bias line 111 and the virtualplane. Note that, in the present embodiment, all the third conductorlayers 110 and the first conductor layer 106 in all the antennas arearranged in a layer different from any of the fourth conductor layers111. Here, the wire constituting the bias line 111 has a part largerthan the width of the wire constituting the coupling line 109. Thelarger part is located, for example, between a plurality of antennas.

In this way, the coupling line 109 that transmits a high frequency(f_(THz)) and the bias line 111 that transmits a low frequency (DC toseveral tens of GHz) are arranged in different layers. This makes itpossible to freely set the layout such as the width, length, and routingof the transmission line in each layer.

Further, in the semiconductor element 100, the substrate 113, the firstconductor layer 106, and the second conductor layer 103 a are stacked inthis order from the substrate 113 side. Also, at least one of thecoupling line 109 and the bias line 111 is disposed in a layer betweenthe first conductor layer 106 and the second conductor layer 103. Forexample, as illustrated in FIG. 1B, the fourth conductor layers 111 x 2and 111 x 1 are arranged in a layer between the first conductor layer106 and the second conductor layer 103.

Further, as illustrated in FIG. 2, when viewed from above (in planview), the coupling line 109 and the bias line 111 cross each other. Forexample, as illustrated in FIGS. 1B and 1C, in plan view, the thirdconductor layer 110 ab and the fourth conductor layer 111 x 2 cross eachother, and the third conductor layer 110 ad and the fourth conductorlayer 111 y 3 cross each other.

In this way, by drawing lines so that the coupling line 109 and the biasline 111 cross each other, a more layout-saving configuration can berealized. Therefore, with such a configuration, the number of antennasto be arranged can be increased even in an antenna array in whichantennas are arranged in a matrix of m×n (m≥2, n≥2). According to thepresent embodiment, even if the number of antennas is increased, it ispossible to suppress the physical interference between the coupling line(coupling line 109) for synchronizing the antennas and the feeding line(bias line 111) for supplying a bias to each RTD 101. Therefore, in thesemiconductor element 100, the restriction of the upper limit of thenumber of antennas to be arranged can be relaxed, and it can be expectedthat an increased number of antennas in the array have a great effect ofimproving the directivity and the front strength.

Further, by disposing at least one of the coupling line 109 and the biasline 111 in a layer between two conductor layers forming the antenna, alayout-saving configuration can be realized. Specifically, the couplingline 109 and/or the bias line 111 are embedded in a free area other thanthe antennas in the dielectric layer 104 forming the antennas 100 a to100 i. As a result, a plurality of transmission lines can be arranged ina relatively small space between adjacent antennas arranged at a pitchof about a wavelength, so that it is possible to sufficiently cope withan increased number of lines due to an increase in the number ofantennas.

Note that, in the terahertz band, the resistance due to the skin effectincreases, so that the conductor loss associated with high-frequencytransmission between antennas is not negligible. FIG. 3 illustrates aresult of analysis of the correlation between a dielectric layerthickness of a microstrip line having a configuration similar to that ofthe present embodiment and the conductor loss at 0.5 THz. The microstriplines used in the analysis has a structure in which a dielectric (SiO₂,ε_(r)=4, tan δ=0) is interposed between an upper conductor layer(material Au, 1 μm thickness, conductivity 2×10⁷ S/m) with a line widthof 10 μm and a grounding conductor (material Au, 1 μm thickness,conductivity 2×10⁷ S/m). For the analysis of the conductor loss, HFSSwas used which is a finite element method solver for high-frequencyelectromagnetic fields, commercially available from ANSYS, Inc.

As the current density between conductor layers increases, the conductorloss (dB/mm) per unit length increases. Further, as illustrated in FIG.3, for the microstrip line, the conductor loss (dB/mm) per unit lengthis inversely proportional to the square of the dielectric thickness.Therefore, in order to increase the radiation efficiency of the antennaarray, it is preferable to thicken not only the antenna but also thedielectric forming the coupling line 109 to reduce the conductor loss.By contrast, the semiconductor element 100 according to the presentembodiment has a configuration in which the bias line 111 is provided onthe first conductor layer 106 side in the first dielectric layer 1041,and the third conductor layer 110 in which a high frequency wave withthe frequency f_(THz) is transmitted is provided as an upper layer ofthe dielectric layer 104. With this configuration, it is possible tosuppress a decrease in the radiation efficiency of the antenna array dueto the conductor loss in the terahertz band. With this configuration forthe antenna 100 a, the substrate 113, the first conductor layer 106, thefourth conductor layers 111 x 1 and 111 x 2, the second conductor layer103 a, and the third conductor layers 110 ad and 110 ab are stacked inthis order from the substrate 113 side. The same applies to therelationship between the coupling line 109 for coupling the otherantennas and the bias line 111.

As described above, the semiconductor element 100 according to thepresent embodiment has a configuration with high radiation efficiency.Note that, from the viewpoint of the conductor loss illustrated in FIG.3, the thickness of the dielectric forming the coupling line 109 ispreferably 1 μm or more. and more preferably, a dielectric thickness setto 2 μm or more makes it possible to reduce losses due to the conductorloss in the terahertz band to about 20%. Similarly, from the viewpointof the conductor loss, it is preferable that the distance in thethickness direction between the third conductor layer 110 and the firstconductor layer 106, which form the coupling line 109 is long. Further,it is preferable that the distance in the thickness direction betweenthe third conductor layer 110 forming the coupling line 109 and thefourth conductor layer 111 forming the bias line 111 is long. The biasline 111 can function as a low impedance line for up to the gigahertzband when the dielectric is set to 2 μm or less, preferably 1 μm orless. Further, even when the dielectric is set to have a thickness of 2μm or more, it can function as a low impedance line as long as it has aconfiguration in which a shunt component is connected to the bias lineas in the semiconductor element 300.

Further, in the semiconductor element 100 according to the presentembodiment, feeding is performed for the adjacent antennas by the commonbias line 111 disposed between the antennas. For example, as illustratedin FIG. 1C, the antenna 100 a is connected to the bias line 111 y 3 viathe conductor 107 a 2 and the line 108 a 2, and the antenna 100 d isconnected to the bias line 111 y 3 via a conductor 107 d 1 and a line108 d 1. Similarly, the antenna 100 a and the antenna 100 b are adjacentto each other, so that a bias signal is fed to them by the connection tothe common bias line 111 x 2 disposed between the two antennas. The sameapplies to the bias lines 111 for the other antennas 100 b to 100 i. Inthis way, the common use of the bias line 111, which is a wire in thechip, among the antennas makes it possible to drive the antennas in thesame channel, resulting in a simplified drive method. Further, thenumber of wires can be reduced and each wire can be made thicker, sothat it is possible to prevent the wiring resistance from increasing dueto an increased number of antennas in the array and accordingly, preventthe operating point from shifting among the antennas. As a result, it ispossible to prevent the frequency and phase from shifting among theantennas due to an increased number of antennas in the array, resultingin more easily obtaining the synchronization effect of the arrays.

Note that the common use of the bias line 111 is not an essentialstructure. For example, for each antenna, a plurality of bias lines 111may be prepared by being multilayered or miniaturized for their separatefeeding. In this case, the isolation between the antennas via the biasline 111 is enhanced, so that the risk of low-frequency parasiticoscillation can be reduced. Further, in the semiconductor element 100,the lines 108 a 1, 108 a 2 to 108 i 1, 108 i 2 and the bias line 111preferably have a lower impedance than the negative resistance of theRTDs 101 a to 101 i in a low frequency band lower than the oscillationfrequency f_(THz). More preferably, the impedance is equal to orslightly smaller than the absolute value of the combined differentialnegative resistances of the RTDs 101 a to 101 i which are connected inparallel. This makes it possible to suppress low-frequency multimodeoscillation.

As described above, according to the present embodiment, it is possibleto reduce the loss of electromagnetic waves as compared with theconventional case, and to produce an oscillating terahertz wave ordetect a terahertz wave with higher efficiency.

(First Example): A specific configuration of the semiconductor element100 that produces an oscillating terahertz wave, according to the firstembodiment, will be described with reference to FIGS. 1A to 1C as afirst example. The semiconductor element 100 is a semiconductor devicecapable of single-mode oscillation in a frequency band of 0.45 to 0.50THz. The RTDs 101 a to 101 i have a multiple quantum well structure inwhich InGaAs/AlAs is lattice-matched on the InP substrate 113, and inthe present example, an RTD having a double barrier structure is used aseach. The semiconductor layer heterostructure of the RTD is thestructure disclosed in J Infrared Milli Terahz Waves (2014) 35: 425-431(NPL 3).

The current-voltage characteristics of the RTDs 101 a to 101 i aremeasured values of a peak current density of 9 mA/μm² and a differentialnegative conductance per unit area of 10 mS/μm². In the antenna 100 a, amesa structure is formed which is composed of the semiconductor layer115 a which includes the RTD 101 a and the electrode 116 a which is anohmic electrode. The mesa structure is circular with a diameter of 2 μmin the present example. At this time, the magnitude of the differentialnegative resistance of the RTD 101 a is about −30Ω per diode. In thiscase, the differential negative conductance (G_(RTD)) of thesemiconductor layer 115 a including the RTD 101 a is estimated to beabout 30 mS, and the diode capacitance (C_(RTD)) of the RTD 101 a isestimated to be about 10 fF.

The antenna 100 a is a patch antenna having a structure in which thedielectric layer 104 is interposed between the second conductor layer103 a, which is a patch conductor, and the first conductor layer 106,which is a grounding conductor. The semiconductor layer 115 a includingthe RTD 101 a is integrated inside the antenna 100 a. The antenna 100 ais a square patch antenna having a side of the second conductor layer103 a of 150 μm, and the resonator length (L) of the antenna is 150 μm.

A metal layer mainly composed of an Au thin film having a lowresistivity is used for the second conductor layer 103 a, which is apatch conductor, and the first conductor layer 106, which is a groundingconductor. The second conductor layer 103 a is made of a metalcontaining Ti/Au (=5/300 nm). The dielectric layer 104 is a layerdisposed between the second conductor layer 103 a and the firstconductor layer 106. The dielectric layer 104 is composed of two layers:the first dielectric layer 1041 made of BCB (benzocyclobutene, availablefrom Dow Chemical Co., Ltd., ε_(r1)=2) with a thickness of 5 μm and thesecond dielectric layer 1042 made of SiO₂ (plasma CVD, ε_(r2)=4) with athickness of 2 μm.

The first conductor layer 106 is composed of a Ti/Pd/Au layer (20/20/200nm) and a semiconductor layer composed of an n⁺-InGaAs layer (100 nm)having an electron concentration of 1×10¹⁸ cm⁻³ or more, and the metaland the semiconductor layer are connected by low resistance ohmiccontact.

The electrode 116 a is an ohmic electrode composed of a Ti/Pd/Au layer(20/20/200 nm). The electrode 116 a is connected to a semiconductorlayer formed on the semiconductor layer 115 a and composed of ann⁺InGaAs layer (100 nm) having an electron concentration of 1×10¹⁸ cm⁻³or more by low resistance ohmic contact.

Around the RTD 101 a, the substrate 113, the first conductor layer 106,the semiconductor layer 115 a, the electrodes 116 a, the conductor 117 acomposed of a conductor containing Cu, and the second conductor layer103 a are stacked in this order from the substrate 113 side, and areelectrically connected. The RTD 101 a is disposed at a position shiftedby 40% (60 μm) of one side of the second conductor layer 103 a in theresonance direction (AA′ direction) from the center of gravity of thesecond conductor layer 103 a. Here, the input impedance when a highfrequency wave is fed from the RTD to the patch antenna is determineddepending on the position of the RTD 101 a in the antenna 100 a. Thesecond conductor layer 103 a is connected to the lines 108 a 1 and 108 a2 arranged in the lower layer via the conductors 107 a 1 and 107 a 2which are vias made of Cu.

The lines 108 a 1 and 108 a 2 are formed of a metal layer containingTi/Au (=5/300 nm) stacked on the second dielectric layer 1042. The lines108 a 1 and 108 a 2 are connected to the bias circuit 120 via the biasline 111 which is a common wire formed in the chip. The bias line 111 isformed of a metal layer containing Ti/Au (=5/300 nm) stacked on thesecond dielectric layer 1042. The antenna 100 a is designed to set abias in the negative resistance region of the RTD 101 a so thatoscillation at a power of 0.2 mW can be obtained at a frequency off_(THz)=0.5 THz.

The conductors 107 a 1 and 107 a 2 have a cylindrical structure with adiameter of 10 μm. The lines 108 a 1 and 108 a 2 are composed of apattern formed of a metal layer containing Ti/Au (=5/300 nm) having awidth of 10 μm in the resonance direction (=A-A′ direction) and a lengthof 75 μm. The conductors 107 a 1 and 107 a 2 are connected to the secondconductor layer 103 a at the center in the resonance direction (=A-A′direction) and at the ends in the B B′ direction. These connectionpositions correspond to the nodes of the electric field of the f_(THz)terahertz wave standing in the antenna 100 a.

The semiconductor element 100 is an antenna array in which the nineantennas 100 a to 100 i are arranged in a 3×3 matrix. Each antenna isdesigned to produces an oscillating terahertz wave at the frequency off_(THz) alone, and is disposed at a pitch (interval) of 340 μm in boththe A-A′ direction and the B-B′ direction. The adjacent antennas aremutually coupled by the coupling line 109 including the third conductorlayer 110 made of Ti/Au (=5/300 nm). For example, the antenna 100 a andthe antenna 100 b are mutually coupled by the coupling line 109 ab. Thesecond conductor layer 103 a and the second conductor layer 103 b aredirectly connected by the third conductor layer 110 ab, having a widthof 5 μm and a length of 190 μm, which is formed in the same layer.Further, the antenna 100 a and the antenna 100 d are mutually coupled bythe coupling line 109 ad. The second conductor layer 103 a and thesecond conductor layer 103 d are directly connected by the thirdconductor layer 110 ad, having a width of 5 μm and a length of 440 μm,which is formed in the same layer. The same applies to other antennas.The antennas 100 a to 100 i are in mutual injection locking in a stateof being in phase with each other (positive phase) at the oscillationfrequency of f_(THz)=0.5 THz to produce oscillating waves.

The bias line 111, which is a common wire formed in the chip, is a biaswire common among the antennas, and is connected to the lines 108 a 1,108 a 2 to 108 i 1, 108 i 2 connected to the antennas 100 a to 100 i.

In the semiconductor element 100, the coupling line 109 and the biasline 111 are arranged in different layers, as with the relationshipbetween the third conductor layer 110 ab of the coupling line 109 ab andthe fourth conductor layer 111 x 1 of the bias line 111. Further, in thesemiconductor element 100, the substrate 113, the first conductor layer106, and the second conductor layer 103 a are stacked in this order fromthe substrate 113 side. Further, the bias line 111 is disposed in alayer between the first conductor layer 106 and the second conductorlayer 103, as with the fourth conductor layer 111 x 1. Further, thecoupling line 109 and the bias line 111 cross each other. The sameapplies to the relationship between the coupling line 109 for couplingthe other antennas 100 b to 100 i and the bias line 111. With such aconfiguration, it is possible to reduce the physical interferencebetween the coupling line (coupling line 109) for synchronizing theantennas and the feeding line (bias line 111) for supplying a bias toeach RTD 101. Therefore, the number of antennas to be arranged isincreased, so that it can be expected that an increased number ofantennas in the array have a great effect of improving the directivityand the front strength.

(Method for Manufacturing Semiconductor Element): Next, a method formanufacturing (method for producing) the semiconductor element 100according to the present example will be described.

(1) First, an InGaAs/AlAs-based semiconductor multilayer film structurethat forms the semiconductor layers 115 a to 115 i including the RTD 101a to 101 i is formed by epitaxial growth on the substrate 113 made ofInP. It is formed by a molecular beam epitaxy (MBE) method, ametalorganic vapor phase epitaxy (MOVPE) method, or the like.

(2) A Ti/Pd/Au layer (20/20/200 nm) that forms the ohmic electrodes 116a to 116 i is formed on the semiconductor layers 115 a to 115 i by asputtering method.

(3) The electrodes 116 a to 116 i and the semiconductor layers 115 a to115 i are formed into a circular mesa shape having a diameter of 2 μm toform a mesa structure. Here, photolithography and ICP (inductivelycoupled plasma) dry etching are used to form the mesa shape.

(4) After the first conductor layer 106 is formed on the substrate 113by a lift-off method on the etched surface, a silicon oxide film havinga thickness of 2 μm to be the second dielectric layer 1042 is formed bya plasma CVD method.

(5) A Ti/Au layer (=5/300 nm) is formed on the second dielectric layer1042 as the fourth conductor layer 111 forming the lines 108 a 1 to i 2and the bias line 111. This completes the formation of the bias line111.

(6) A BCB film with a thickness of 5 μm to be the first dielectric layer1041 is embedded and flattened using a spin coating method and a dryetching method.

(7) Parts of the BCB and silicon oxide films which form the conductors117 a to 117 i and the conductors 107 a 1 to 107 i 2 to be vias areremoved by photolithography and dry etching to form via holes (contactholes). Also, using photolithography including grayscale exposure forthis formation makes it possible to optionally control the taper angleof the via holes for forming the first dielectric layer 1041 and thesecond dielectric layer 1042 and the coupling line 109.

(8) The conductors 117 a to 117 i and conductors 107 a 1 to 107 i 2,which are vias, are formed by conductors containing Cu in the via holes.For the formation of the conductors 117 a to 117 i and the conductors107 a 1 to 107 i 2, via hole embedding and flattening with Cu arecarried out by using a sputtering method, an electroplating method, anda chemical mechanical polishing method.

(9) Electrode Ti/Au layer films (=5/300 nm) to be the second conductorlayers 103 a to 103 i of the antennas and the third conductor layer 110forming the coupling line 109 are formed by a sputtering method.

(10) By photolithography and ICP (inductively coupled plasma) dryetching, the second conductor layers 103 a to 103 i and the thirdconductor layer 110 forming the coupling line 109 are patterned. Thisstep completes the formation of the coupling line 109.

(11) Finally, the shunt resistor 121 and the MIM capacitor 124 areformed, and they are connected to the wire 122 and the power supply 123by wire bonding or the like to complete the semiconductor element 100.

Note that the power supply to the semiconductor element 100 is performedby the bias circuit 120, and normally, when a bias voltage resulting ina differential negative resistance region is applied to supply a biascurrent, the semiconductor element 100 operates as an oscillator.

<Second Embodiment> FIGS. 4A, 4B, and 4C illustrate a semiconductorelement 200 according to a second embodiment. Note that theconfigurations and structures of the semiconductor element 200 otherthan the following have the same components having the same name of thesemiconductor element 100 according to the first embodiment, anddetailed description thereof will be omitted accordingly. Further, alsoin the present embodiment, a coupling line 209 and a bias line 211 arearranged in different layers as in the first embodiment.

The semiconductor element 200 is an antenna array in which nine antennas200 a to 200 i are arranged in a 3×3 matrix. Unlike the firstembodiment, the antenna 200 a includes two active layers having anelectromagnetic wave gain or non-linearity for a terahertz wave in oneantenna. Specifically, the antenna 200 a includes a semiconductor layer215 a 1 including an RTD 201 a 1 and a semiconductor layer 215 a 2including an RTD 201 a 2.

An electrode 216 a 1 and an electrode 216 a 2 are arranged on the sideopposite to the side where a first conductor layer 206 is disposed withrespect to the semiconductor layer 215 a 1 and the semiconductor layer215 a 2. The electrode 216 a 1 and the semiconductor layer 215 a 1 areelectrically connected, and the electrode 216 a 2 and the semiconductorlayer 215 a 2 are electrically connected. Further, a bias signal is fedfrom the bias circuit 120 to the two RTDs 201 a 1 and 201 a 2 via theconductor layers 217 a 1 and 217 a 2, which are vias connected between asecond conductor layer 203 a and the electrodes 216 a 1 and 216 a 2.

The RTD 201 a 1 is disposed at a position shifted by 40% of the lengthof one side of the second conductor layer 203 a in the resonancedirection (i.e., AA′ direction) from the center of gravity of the secondconductor layer 203 a. On the other hand, the RTD 201 a 2 is disposed ata position shifted by −40% of the length of one side of the secondconductor layer 203 a in the resonance direction (i.e., AA′ direction)from the center of gravity of the second conductor layer 203 a. In otherwords, the RTD 201 a 1 and the RTD 201 a 2 are arranged at positionsthat pass through the center of gravity of the second conductor layer203 a and are line-symmetrical with respect to a straight line (centerline) perpendicular to the resonance direction and the stackingdirection. With this configuration, the RTD 201 a 1 and RTD 201 a 2 arein mutual injection locking in a state of being in phase inverted witheach other (negative phase) to produces oscillating waves. In this way,the vertically and horizontally symmetrical configuration of the RTDs inthe antenna is a configuration in which an increased number of antennasin the array easily have an effect of improving the directivity and thefront strength.

The coupling line 209 is composed of a microstrip line in which adielectric layer 204 and a dielectric layer 217 are interposed betweenthe fourth conductor layer 210 stacked on the dielectric layer 217stacked on the dielectric layer 204 and the first conductor layer 206.For example, as illustrated in FIG. 4B, a coupling line 209 ab has astructure in which the dielectric layer 204 and the dielectric layer 217are interposed between a fourth conductor layer 210 ab and the firstconductor layer 206.

Similarly, a coupling line 209 bc has a structure in which thedielectric layer 204 and the dielectric layer 217 are interposed betweena fourth conductor layer 210 bc serving as an upper conductor layer andthe first conductor layer 206, and a coupling line 209 ad has astructure in which the dielectric layer 204 and the dielectric layer 217are interposed between a fourth conductor layer 210 ad serving as anupper conductor layer and the first conductor layer 206.

The semiconductor element 200 is an antenna array having a configurationin which antennas are coupled by AC coupling (capacitive coupling). Forexample, the fourth conductor layer 210 ab, which is the upper conductorlayer in the coupling line 209 ab for coupling the antenna 200 a and theantenna 200 b, overlaps the second conductor layers 203 a and 203 b by 5μm near the radiation end in plan view. The same applies to the couplingbetween the adjacent antennas of the other antennas 200 b to 200 i.

In the part where the conductor layers overlap, the second conductorlayers 203 a and 203 b, the dielectric layer 217, and the fourthconductor layer 210 ab are stacked in this order to form ametal-insulator-metal (MIM) capacitor structure. With this structure,the relationship between the second conductor layer 203 a and the secondconductor layer 203 b is open for DC, and their coupling degree is smallin a low frequency range below f_(THz), so that isolation between theelements is ensured. On the other hand, in the band of the oscillationfrequency f_(THz), the degree of the coupling between the antennas canbe adjusted by the capacitance. Such a structure makes it possible tosignificantly weaken the coupling between the antennas, which also leadsto the suppression of transmission loss between the antennas, and as aresult, it can be expected to improve the radiation efficiency of theantenna array.

<Third Embodiment>: FIGS. 5A, 5B, and 5C illustrate a semiconductorelement 300 according to a third embodiment. Note that theconfigurations and structures of the semiconductor element 300 otherthan the following have the same components having the same name of thesemiconductor element 200 according to the second embodiment, anddetailed description thereof will be omitted accordingly. Further, alsoin the present embodiment, a coupling line 309 and a bias line 311 arearranged in different layers as in the first embodiment.

The semiconductor element 300 is an antenna array in which nine antennas300 a to 300 i are arranged in a 3×3 matrix. As with the semiconductorelement 200 according to the second embodiment, each of the antennas 300a to 300 i includes two active layers having an electromagnetic wavegain or non-linearity for a terahertz wave in one antenna. Further,unlike the semiconductor element 200 according to the second embodiment,in the semiconductor element 300, the bias line 311 has a shuntstructure in order to suppress parasitic oscillation in a frequency bandlower than the oscillation frequency f_(THz). The shunt structure is astructure that is arranged in parallel with an RTD, which is a negativeresistance element, to short a frequency band lower than f_(THz) so thatparasitic oscillation can be suppressed. Further, the shunt structure isa structure in which a resistance element or elements in which aresistor and a capacitor are connected in series are arranged inparallel with an RTD. In the shunt structure, the resistor and capacitorhave a value such that the impedance of the elements is equal to orslightly lower than the absolute value of the combined differentialnegative resistance of a plurality of RTDs arranged in the vicinity ofthe elements.

The semiconductor element 300 includes, as a dielectric layer 304, threedielectric layers: a first dielectric layer 3041, a second dielectriclayer 3042, and a third dielectric layer 3043. Note that, for the thirddielectric layer 3043 to be used as a dielectric for the capacitor ofthe shunt structure, silicon nitride (ε_(r2)=7) having a relatively highdielectric constant is used to miniaturize the MIM capacitor structure.Here, for the dielectric layer 304 having the three-layer structure, theeffective relative dielectric constant is determined in consideration ofthe thickness and the relative dielectric constant of the thirddielectric layer 3043.

Further, a fifth conductor layer 318 is stacked on the third dielectriclayer 3043. Accordingly, from a substrate 313 side, ametal-insulator-metal (MIM) capacitor structure is formed in which afirst conductor layer 306, the third dielectric layer 3043, and thefifth conductor layer 318 are stacked in this order, and this capacitorstructure is disposed under the bias line 311. The fifth conductor layer318 is disposed in a layer between the first conductor layer 306 andboth a third conductor layer 310 and the fourth conductor layer 311.

Here, each of the antennas 300 a to 300 i has a resistor and a capacitoras the shunt structure. For example, for the antenna 300 a, a resistiveelement 319 y 4 a connected to a bias line 311 y 4 corresponds to theresistor. The MIM capacitor structure in which the third dielectriclayer 3043 is interposed between a fifth conductor layer 318 y 4 aconnected to the resistive element 319 y 4 a and the first conductorlayer 306 corresponds to the capacitor. As described above, in thepresent embodiment, the first conductor layer 306 and the bias line 311are electrically connected via the capacitor and the resistor. Notethat, depending on the arrangement of the bias line 311 and the thirdconductor layer 310, the first conductor layer 306 and the thirdconductor layer 310 may be electrically connected via a resistor.

Further, a fifth conductor layer 318 y 3 ad is connected to a resistiveelement 319 y 3 ad connected to the bias line 311 y 3. The thirddielectric layer 3043 is interposed between the fifth conductor layer318 y 3 ad and the first conductor layer 306, which is the MIM capacitorstructure. Note that the configuration in which the shunt structure isdisposed in a node of the high frequency electric field having theoscillation frequency f_(THz) standing in the antennas 300 a to 300 ihas high impedance at the frequency f_(THz), and thus it is moresuitable for selectively producing only a high frequency oscillatingwave at the frequency f_(THz).

However, there is a risk that unexpected low-frequency multimodeoscillation will occur due to an increased number of antennas in thearray and the common use of the bias line in the antenna array.Therefore, the semiconductor element 300 has a configuration in whichlines 308 a 1, 308 a 2 to lines 308 i 1, 308 i 2 and the bias line 311are set to have a lower impedance than the negative resistance element(semiconductor layer 315) in a low frequency band lower than theoscillation frequency f_(THz). With this structure, even for anincreased number of antennas in the antenna array, it is possible tosuppress other mode oscillation and obtain stable single frequencyoscillation in the terahertz band.

<Fourth Embodiment>: FIGS. 6A to 6C, FIG. 7, and FIGS. 8A to 8Cillustrate a semiconductor element 400 and a semiconductor element 500,according to a fourth embodiment. The semiconductor elements 400 and 500are each an antenna array in which nine antennas are arranged in a 3×3matrix. Here, coupling lines 409 and 509 are each disposed in a lowerlayer, and bias lines 411 and 511 are each disposed in an upper layer.The coupling line needs to be phase-matched between the antennas in theterahertz band, and thus the shape may be complicated depending on theantenna configuration. On the other hand, the bias line for bias feedingcan be a relatively simple pattern. Therefore, the configuration inwhich the bias line is disposed in the upper layer and the coupling lineis disposed in the lower layer as in the present embodiment is aconfiguration in which the interference between the metal body otherthan the antenna and the emitted electromagnetic wave is reduced.

The semiconductor element 400 illustrated in FIGS. 6A to 6C includesnine antennas of antennas 400 a to 400 i. As illustrated in FIG. 6B, athird conductor layer 410 ab is disposed below a fourth conductor layer411 x 2 that is stacked on a first dielectric layer 4041 and that formsthe bias line 411. Note that the third conductor layer 410 ab is stackedon the first dielectric layer 4041 forming the coupling line 409 abassociated with the antenna 400 a. The same applies to the otherantennas 400 b to 400 i as well as the antenna 400 a.

Accordingly, in the semiconductor element 400, a substrate 413, a firstconductor layer 406 which is a grounding conductor for the antenna, thethird conductor layer 410 ab, a second conductor layer 403 a which is apatch conductor, and the fourth conductor layer 411 x 2 are stacked inthis order from the substrate 413 side. The adjacent antennas of thesemiconductor element 400 are coupled by DC coupling. For example, thethird conductor layer 410 ab, which is the upper conductor layer of thecoupling line 409 ab for coupling the antenna 400 a and the antenna 400b, is directly connected to the second conductor layers 403 a and 403 b.The same applies to the coupling between the other antennas. Here, inthe semiconductor element 400, the third conductor layer 410 ab isformed in the lower layer of the second conductor layers 403 a and 403 bso as to be covered with the first dielectric layer 4041. Note that, inthe present embodiment, the second conductor layer 403 and the fourthconductor layer 411 are arranged in the same layer, but theirarrangement is not limited to this, and the fourth conductor layer 411may be formed in a lower layer of the second conductor layer 403.

The semiconductor element 500 illustrated in FIGS. 7 to 8C includes nineantennas of antennas 500 a to 500 i. The semiconductor element 500 is anantenna array in which the adjacent antennas are connected by a couplingline 509 which is a microstrip line including a third conductor layer510 serving as an upper conductor layer. Note that the third conductorlayer 510 is disposed between a first conductor layer 506, which is agrounding conductor, and second conductor layers 503 a to 503 i, whichare patch conductors.

In the antenna 500 a, a composite resonator including a patch antennaand a coupling line 509 a is integrated in an RTD 501 a, and the patchantenna is composed of the first conductor layer 506 and the secondconductor layer 503 a, and the coupling line 509 a is composed of thefirst conductor layer 506 and a third conductor layer 510 a.

The coupling line 509 a has a structure in which a second dielectriclayer 5042 is interposed between the first conductor layer 506 and thethird conductor layer 510 a, and the longitudinal direction of thecoupling line 509 a is a direction (i.e., CC′ direction) perpendicularto the resonance direction (i.e., AA′ direction). The third conductorlayer 510 a is connected to a via 517 a connecting the second conductorlayer 503 a and the RTD 501 a. As a result, the RTD 501 a is coupled tothe two resonators: the patch antenna defined by the second conductorlayer 503 a and the coupling line 509 a defined by the third conductorlayer 510 a. Accordingly, the length of the coupling line 509 a and thesize of the patch antenna are important parameters for determining thefrequency of an electromagnetic wave to oscillate. The oscillationfrequency f_(THz) of the antenna 500 a can be determined by the lengthof the second conductor layer 503 a in the AN direction and the lengthof the third conductor layer 510 a in the CC′ direction. Specifically,the length of the third conductor layer 510 a in the CC′ direction maybe set to an integral multiple of the effective length of the desiredoscillation wavelength, and the length of the second conductor layer 503a in the AA′ direction may be set to ½ of the effective length of thedesired oscillation wavelength. Here, a bias line 511 is composed of afourth conductor layer 511 y 3 stacked on a first dielectric layer 5041,and the third conductor layer 510 a is disposed below the fourthconductor layer 511 y 3. The same applies to the components of the otherantennas 500 b to 500 i. Note that, in the present embodiment, thesecond conductor layer 503 and the fourth conductor layer 511 arearranged in the same layer, but their arrangement is not limited tothis, and the fourth conductor layer 511 may be formed in a lower layerof the second conductor layer 503.

The adjacent antennas are connected by the coupling line 509 by DCcoupling. For example, for the coupling of the antenna 500 a and theantenna 500 b, they are directly connected to each other by a thirdconductor layer 510 abde at the end of the third conductor layers 510 aand 510 b which are the upper conductor layers of the coupling lines 509a and 509 b. Further, for the coupling between the antenna 500 a and theantenna 500 d, they are directly connected to each other at the end ofthe third conductor layers 510 a and 510 d which are the upper conductorlayers of the coupling lines 509 a and 509 d. Note that it is preferableto locate the RTDs 501 a to 501 i at the maximum points of the electricfield of the electromagnetic wave (oscillation frequency f_(THz))standing in the coupling line 509 in order to enhance the entrainmentsynchronization between the antennas. The same applies to the couplingbetween the other antennas.

As described above, the third conductor layer 510 of the coupling line509 is in a layer different from the second conductor layer 503 formingthe patch antenna, thereby increasing the degree of freedom of devicedesign for performing phase synchronization in the array.

<Fifth Embodiment>: An oscillating element 1000 that is a semiconductorelement according to the fifth embodiment will be described withreference to FIGS. 9A and 9B. FIG. 9A is a top view of the oscillatingelement 1000 having 2×2 antennas. FIG. 9B is a cross-sectional viewtaken along the line B-B′ indicated in FIG. 9A. Note that, also in thepresent embodiment, a coupling line and a bias line are arranged indifferent layers.

The oscillating element 1000 includes a substrate 1001, a firstconductor layer 1002 (ground metal (GND)), a negative resistance element1003, a second dielectric layer 1004, and a third conductor layer 1005(microstrip line (MSL) coupling line). The oscillating element 1000 alsoincludes a first dielectric layer 1006 and a second conductor layer1007.

In the present embodiment, the second dielectric layer 1004 isinterposed between the third conductor layer 1005 and the firstconductor layer 1002 to form the coupling line.

Further, the oscillating element 1000 may have a shunt structure 1008(filter unit), but the shunt structure 1008 is not an essentialconfiguration. The shunt structure 1008 has a capacitor part 1009 and aresistor part 1010. The capacitor part 1009 has a MIM structure in whicha high dielectric constant layer 1011 is interposed between a conductivesubstrate 1001 and a conductor layer which is formed at the same timewhen the first conductor layer 1002 is formed.

In the present embodiment, the third conductor layer 1005 forsynchronizing the second conductor layer 1007 with the phase of theadjacent antenna is formed in a layer different from the secondconductor layer 1007, thereby increasing the degree of freedom of designand thus making it possible to arrange a plurality of antennas in anarray.

As the substrate 1001, an n⁺InP substrate is used. In the substrate1001, the InP substrate includes a semiconductor multilayer film thatproduces a terahertz wave, and has an electromagnetic wave gain in afrequency range of terahertz waves.

As the negative resistance element 1003, for example, a resonance tunneldiode (RTD) or a Gunn diode can be used, and in the present embodiment,the negative resistance element 1003 is formed of an RTD.

The substrate 1001 is connected to the first conductor layer 1002 byohmic contact, and on the cathode side, a structure is used in which thefirst conductor layer 1002 is connected to the negative resistanceelement via the substrate 1001. On the anode side, the bias line isconnected to the second conductor layer 1007, and the second conductorlayer 1007 is connected to the negative resistance element 1003 via thethird conductor layer 1005. Thus, in the present embodiment, the biasline (fourth conductor layer) is formed in the same layer as the secondconductor layer 1007. Applying a bias to the negative resistance element1003 makes it possible to obtain terahertz wave oscillation by thesecond conductor layer 1007, the negative resistance element 1003, andthe third conductor layer 1005, which act as a resonator.

In order to control the phases of the electromagnetic waves oscillatingat the adjacent antennas, the negative resistance element 1003 isconnected between the adjacent antennas by the third conductor layer1005 (MSL coupling line). Here, the negative resistance element 1003(antenna) is located at the maximum point of the electric field of theelectromagnetic wave standing in the third conductor layer 1005, so thatthe phases of the electromagnetic waves produced at the antennas aresynchronized.

Further, the length of the third conductor layer 1005 and the size ofthe second conductor layer 1007 are important parameters for determiningthe frequency of an electromagnetic wave to oscillate. The length of thethird conductor layer 1005 (MSL coupling line) in the resonancedirection may be set to an integral multiple of the effective wavelengthλ of the desired oscillation wavelength, and the length of the secondconductor layer 1007 in the resonance direction may be set to ½ of theeffective length of the desired oscillation wavelength λ.

In FIG. 9A, the antennas of the oscillating element 1000 are arranged in2 rows×2 columns. Here, the third conductor layer 1005 is wired so thatL1=effective wavelength λ and L2=λ/2, and the negative resistanceelement 1003 is located at a distance of λ/2 from the distal end of thethird conductor layer 1005. Further, the negative resistance elements1003 of the adjacent antennas are located at the maximum points of theelectric field of the standing electromagnetic wave as described above,that is, at a distance of an integral multiple of λ from each other. Inother words, in the oscillating element 1000, the antennas are arrangedat a pitch that is an integral multiple of λ.

In the present embodiment, since the RTD element having a gain from alow frequency to about 2 THz is used as the negative resistance element1003, oscillation (parasitic oscillation) at a frequency other than thedesired frequency may occur. In this respect, it is preferable to form afilter in order to suppress parasitic oscillation. For example, in orderto suppress parasitic oscillation, there is known a method ofsuppressing parasitic oscillation by inserting a resistor having aresistance value not more than the absolute value of the negativeresistance into the minimum point of the current of the electromagneticwave standing in the third conductor layer 1005 to cause a loss inelectromagnetic waves with a frequency other than the desired frequency.In the present embodiment, as illustrated in FIGS. 9A and 9B, the shuntstructure 1008 using a λ/4 wire, MIM capacitor and resistor is located.The λ/4 wire is connected to the minimum point of the electric field ofthe electromagnetic wave standing in the third conductor layer 1005, andis connected to the first conductor layer 1002 via the resistor part1010 having a resistance value not more than the absolute value of thenegative resistance and the capacitor part 1009 having a sufficientlyhigh capacitance. At the part of the shunt structure 1008 in contactwith the third conductor layer 1005, the impedance becomes high at thedesired oscillation frequency, making it difficult for a current to flowinto the shunt structure 1008. However, the impedance becomes low at afrequency other than the desired oscillation frequency, making it easy acurrent to flow through the shunt structure and thus causing a loss inthe resistor part 1010, so that parasitic oscillation can be suppressed.In this way, stable oscillation can be obtained by properly locating thefilter.

By connecting to the adjacent antenna by the third conductor layer 1005formed between the first conductor layer 1002 and the second conductorlayer 1007 in this way, the antennas can be arranged in an array. As aresult, the antennas can be arranged at the pitch of effectivewavelength, and the directivity of the electromagnetic wave can beimproved.

(Second Example ): A specific second example of the oscillating element1000 according to the fifth embodiment will be described with referenceto FIGS. 10, 11A and 11B. FIG. 10 is a top view of one antenna includedin the oscillating element 1000. FIG. 11A is a cross-sectional viewtaken along the C-C′ section indicated in FIG. 10. FIG. 11B is a topview in which antennas are arranged in a 4×4 array. For simplification,the substrate 1001 and the first conductor layer 1002 are notillustrated in FIGS. 10 and 11B, and the shunt structure 1008 is notillustrated in FIG. 11B.

In the present example, the oscillating element 1000 of the 4×4 antennaillustrated in FIG. 11B is realized by arranging the antennas eachcorresponding to the single antenna illustrated in FIG. 10 in an arrayat intervals of the effective wavelength λ. Note that the oscillatingelement 1000 having m×n antennas (m and n are integers) can be realizedby arranging the antennas in the same manner as illustrated in FIG. 11B.

First, one antenna illustrated in FIG. 10 will be described. Here,important parameters for determining the oscillation frequency of theoscillating element include, for example, the length of the secondconductor layer 1007 in the resonance direction and the length of thethird conductor layer 1005 in the resonance direction, as well as thedielectric constant of each dielectric layer. Note that, in the presentexample, silicon dioxide formed by plasma CVD is used for the seconddielectric layer 1004. Further, BCB (benzocyclobutene) is used for thefirst dielectric layer 1006.

As a result of calculation using the electromagnetic field simulatorHFSS commercially available from ANSYS, Inc. in order to estimate anoscillation frequency for the present example, the effective wavelengthλ for obtaining an oscillation frequency of 500 GHz was 320 μm.Therefore, the length of the second conductor layer 1007 in theresonance direction may be set to 160 μm which is ½ of the effectivewavelength λ, and the length of the third conductor layer 1005 in theresonance direction may be set to 320 μm which is the effectivewavelength λ.

FIG. 12 illustrates a plot of the relationship between the length of thethird conductor layer 1005 (MSL) in the resonance direction and theactual oscillation frequency. In this plot, the length of the patchantenna in the resonance direction is ½ of the length of the thirdconductor layer 1005 in the resonance direction. When the length of thethird conductor layer 1005 in the resonance direction is 320 μm, theresulting oscillation frequency is 460 GHz to 479 GHz, which is close tothe above-mentioned 500 GHz.

Further, important parameters for determining the oscillation output ofthe oscillating element include, for example, a parasitic capacitancebetween a set of the substrate 1001 and the first conductor layer 1002and a set of the third conductor layer 1005 and the second conductorlayer 1007, as well as the characteristics of the negative resistanceelement 1003. Further, the position of the negative resistance element1003 in the second conductor layer 1007 is also included in theimportant parameters. In the present example, the area of the thirdconductor layer 1005 and the dielectric constant and film thickness ofthe second dielectric layer 1004 are significant influential factors.Therefore, in order to reduce the parasitic resistance, the width of theMSL coupling line is made as narrow as possible, and the film thicknessof the second dielectric layer 1004 is made thick.

Further, the impedance of the negative resistance element 1003 used inthe present example is 50 to 60Ω. Here, in order to make the impedancematching between the second conductor layer 1007 and the negativeresistance element 1003, the feeding point impedance of the secondconductor layer 1007 matches the impedance of the negative resistanceelement 1003.

FIG. 13A illustrates a calculation of the influence of the thickness ofthe silicon dioxide film forming the second dielectric layer 1004 on theoscillation output. FIG. 13B illustrates a calculation of the influenceof the width of the third conductor layer 1005 (MSL) on the oscillationoutput. Further, FIG. 13C illustrates a calculation of the influence ofthe distance between the negative resistance element 1003 and the centerof the second conductor layer 1007 on the oscillation output. In thepresent example, the thickness of the silicon dioxide film was set to 2μm, the width of the third conductor layer 1005 was set to 4 μm, and thevalue obtained by dividing the distance between the negative resistanceelement 1003 and the center of the second conductor layer 1007 by theeffective wavelength was set to 15%.

Further, in order to realize the oscillating element 1000 of the presentexample, parasitic oscillation at a frequency other than the desiredoscillation frequency is suppressed. Accordingly, the shunt structure1008 is located so as not to cause a loss in an electromagnetic wave ina state of oscillating at a desired oscillation frequency, and to notproduce oscillating electromagnetic waves with a loss at a frequencyother than the desired frequency. As described above, in the presentexample, in order to suppress parasitic oscillation, a MIM capacitorwith a sufficiently high capacitance is connected to the third conductorlayer 1005 via a resistor of 20Ω, which is an absolute value of thenegative resistance of the negative resistance element 1003 of 50Ω orless, and a wire with a λ/4 length. The location point of the 214 lengthwire in the third conductor layer 1005 is connected to a node of theelectric field of the electromagnetic wave standing at the desiredfrequency. In other words, in the present example, it is connected at adistance of λ/4 from the distal end of the third conductor layer 1005.As a result, parasitic oscillation can be suppressed and an oscillatingelement with a desired oscillation frequency can be obtained.

Further, as illustrated in FIG. 11A, the central axis of a conductor1012 and the central axis of a contact hole 1013 are displaced from eachother in the stacking direction. The conductor 1012 is an electrodeformed in a contact hole in which the second dielectric layer 1004 onthe negative resistance element 1003 is removed, for electricallyconnecting the negative resistance element 1003 and the third conductorlayer 1005. The contact hole 1013 is a hole that penetrates the seconddielectric layer 1004 (a part of the dielectric layer in which the firstdielectric layer 1006 and the second dielectric layer 1004 are combined)in the stacking direction. By forming a part of the second conductorlayer 1007 on the surface of the contact hole 1013, the third conductorlayer 1005 and the second conductor layer 1007 are connected to eachother. Note that the central axes of the conductor 1012 and the contacthole 1013 may be aligned with each other. However, in the presentexample, in order to protect the negative resistance element 1003, thetwo central axes are displaced from each other so that an upper part ofthe negative resistance element 1003 (in the stacking direction) iscovered with the first dielectric layer 1006 which is a protective film.

By setting the parameters in this way, an output of 460 GHz, 50 μW wasobtained with the single antenna of the present example.

Further, as illustrated in FIG. 11B, the oscillating element 1000 having4×4 antennas can be realized by arranging the antennas corresponding tothe above-mentioned single antenna in an array at the pitch of effectivewavelength.

FIG. 14 illustrates oscillation outputs when a voltage is applied to theoscillating element 1000. In FIG. 14, the broken line is an oscillationoutput obtained by one antenna, and the solid line is an oscillationoutput obtained by 4×4 (16) antennas. Here, for the 4×4 antennas, theoscillation output is 830 μW, and the oscillation frequency is 458 GHz,which is about 16 times the output obtained by the single antenna. Bythe third conductor layer 1005 formed between the second conductor layer1007 and a set of the substrate 1001 and the first conductor layer 1002in this way, the adjacent antennas are connected and arranged in anarray, so that the phases of the electromagnetic waves oscillating atthe antennas can be synchronized.

(Manufacturing Method): Next, a method for manufacturing (method forproducing) of the oscillating element 1000 according to the presentexample will be described with reference to a flowchart of manufacturingsteps of FIG. 15 and FIGS. 16A to 16H. Here, FIGS. 16A to 16H arecross-sectional views of the antenna (oscillating element 1000) in therespective manufacturing steps, and each illustrate a cross-sectionalview taken along the C-C′ in FIG. 10.

In S2001, as illustrated in FIG. 16A, the negative resistance element1003 is formed. More specifically, a semiconductor multilayer filmformed by epitaxial growth on an InP substrate doped with a highconcentration of dopant and an electrode for ohmic contact are processedinto a mesa shape. The semiconductor multilayer film including thenegative resistance element 1003 is formed of InGaAs, AlAs, or the like.The electrode for contact is formed of a metal such as Mo, W, Ti, Ta,Al, Cu, and Au, an alloy thereof, a semiconductor doped with the sameconcentration, and a laminated film thereof. The subsequent processingmethod uses known semiconductor device steps. In the present example, aMo electrode is formed by sputtering on a semiconductor multilayer filmformed by epitaxial growth, a resist having a desired shape is formed bya photolithography step, and then dry etching is performed withchlorine-based gas.

In S2002, as illustrated in FIG. 16B, the high dielectric constant layer1011, which is a MIM capacitor, is formed. In order to reduce the areaof the MIM capacitor, it is desirable that the high dielectric constantlayer 1011 is made of a high dielectric constant material such assilicon nitride and aluminum oxide. In the present example, a siliconnitride film is formed by plasma CVD, a resist having a desired shape isformed by a photolithography step, and then dry etching is performedwith fluorine-based gas.

In S2003, as illustrated in FIG. 16C, the first conductor layer 1002 isformed. The first conductor layer 1002 is formed of a metal such as Mo,W, Ti, Ta, Al, Cu, and Au, an alloy thereof, a semiconductor doped withthe same concentration, and a laminated film thereof. At this time, anupper electrode of the MIM capacitor is also formed at the same time asthe first conductor layer 1002. The first conductor layer 1002 is formedso as to make ohmic contact with the substrate 1001. In the presentexample, a Mo electrode is formed by sputtering, a resist having adesired shape is formed by a photolithography step, and then dry etchingis performed with chlorine-based gas. Further, although not illustrated,the mesa structure is protected by the high dielectric constant layerformed in S2002 during etching of S2003.

In S2004, as illustrated in FIG. 16D, the second dielectric layer 1004is formed. It is desirable that the second dielectric layer 1004 is madeof a low dielectric constant material such as silicon dioxide, BCB,acrylic resin, and polyimide in order to reduce the parasiticcapacitance between a set of the substrate 1001 and the first conductorlayer 1002 and the third conductor layer 1005. In the present example, asilicon dioxide film is formed by plasma CVD, a resist having a desiredshape is formed by a photolithography step, and then dry etching isperformed with fluorine-based gas. By this dry etching, a contact holefor forming the conductor 1012 is formed in the next step.

In S2005, as illustrated in FIG. 16E, the conductor 1012 and the thirdconductor layer 1005 (MSL) are formed. The third conductor layer 1005 isformed of a metal such as Mo, W, Ti, Ta, Al, Cu, and Au, an alloythereof, a semiconductor doped with the same concentration, and alaminated film thereof. In the present example, an Au/Ti laminatedelectrode is formed by sputtering, a resist having a desired shape isformed by a photolithography step, and then wet etching is performed.

In S2006, as illustrated in FIG. 16F, the resistor part 1010 is formed.As described above, the resistance of the resistor part 1010 ispreferably not more than the absolute value of the negative resistanceof the negative resistance element 1003. In the present example, a WTialloy is used for the resistor part 1010 in order to set the resistanceof the resistor part 1010 to several Ω to several tens of Ω. Examples ofother materials that can be used for the resistor part 1010 include ametals such as Ti, TiN, Ta, Mo, and W, an alloy thereof, a semiconductordoped with the same concentration, and a laminated film thereof. In thepresent example, a WTi electrode is formed by sputtering, a resisthaving a desired shape is formed by a photolithography step, and thendry etching is performed with fluorine-based gas.

In S2007, as illustrated in FIG. 16G, the first dielectric layer 1006 isformed. It is desirable that the first dielectric layer 1006 is made ofa low dielectric constant material such as silicon dioxide, BCB, acrylicresin, and polyimide in order to reduce the parasitic capacitancebetween a set of the substrate 1001 and the first conductor layer 1002and the second conductor layer 1007. Further, the contact hole 1013 isalso formed at this time, but it is formed so that the central axis ofthe contact hole 1013 and the central axis of the conductor 1012 are notaligned in the stacking direction. In other words, the contact hole 1013is formed so that a part in the stacking direction of the conductor 1012is maintained in a state of being covered with the first dielectriclayer 1006. In the example, a photosensitive BCB was formed by coating,and a desired shape of the first dielectric layer 1006 was obtained by aphotolithography step.

In S2008, as illustrated in FIG. 16H, the second conductor layer 1007 isformed. The second conductor layer 1007 is formed of a metal such as Mo,W, Ti, Ta, Al, Cu, and Au, an alloy thereof, a semiconductor doped withthe same concentration, and a laminated film thereof. In the presentexample, an Au/Ti laminated electrode is formed by sputtering, a resisthaving a desired shape is formed by a photolithography step, and thenwet etching is performed.

By forming antennas in this way, arranging them in an array, andconnecting the resulting elements by the third conductor layer 1005(MSL), the phases of the electromagnetic waves produced at the antennascan be synchronized.

(Third Example): A third example, which is a modification of the secondexample, will be described with reference to FIG. 17A. Forsimplification, in FIG. 17A, the substrate 1001 and the first conductorlayer 1002 and the shunt structure 1008 are not illustrated. Further,each antenna of the present example has the same configuration as thatof the second example. The present example is an example different fromthe second example in the antenna connection method.

For the connection between the antennas, the connection in the directionof rows is the same as in FIG. 11B of the second example. On the otherhand, for the connection in the direction of columns, the adjacentantennas arranged in the direction of columns at the pitch of effectivewavelength λ are mutually coupled by an electromagnetic wave propagatingthrough a space or an insulating film, instead of the connection usingthe third conductor layer 1005. By arranging the antennas in an array inthis way, and connecting the resulting elements by the third conductorlayer 1005, the phases of the electromagnetic waves produced at theantennas can be synchronized. As a result, the antennas can be arrangedat the pitch of effective wavelength, and the directivity of theelectromagnetic wave can be improved.

(Fourth Example): A fourth example, which is a modification of the thirdexample, will be described with reference to FIG. 17B. Forsimplification, in FIG. 17B, the substrate 1001 and the first conductorlayer 1002 and the shunt structure 1008 are not illustrated. The presentexample is an example of a connection method for one antenna having aplurality of negative resistance elements. In the present example, as inthe third embodiment, for the connection in the direction of columns,the adjacent antennas arranged in the direction of columns at the pitchof effective wavelength λ are mutually coupled by an electromagneticwave propagating through a space or an insulating film, instead of theconnection using the third conductor layer 1005. In the present example,two negative resistance elements are located in one patch antenna, andthey are driven in a state where their phases are inverted from eachother.

By arranging the antennas, in which a plurality of negative resistanceelements are located, in an array in this way, and connecting theresulting elements by the third conductor layer 1005, the phases of theelectromagnetic waves produced at the antennas can be synchronized. As aresult, the antennas can be arranged at the pitch of effectivewavelength, and the directivity of the electromagnetic wave can beimproved.

(Other Embodiments): Although the preferred embodiments of the presentinvention have been described above, the present invention is notlimited to these embodiments, and various modifications and changes canbe made within the scope and spirit of the invention.

For example, in the above-described embodiments and examples, the casewhere the carrier is an electron is described, but the present inventionis not limited to this, and a hole may be used. Further, the materialsof the substrate and the dielectric may be selected according to theapplication, and semiconductor layers such as silicon, gallium arsenide,indium arsenide, and gallium phosphide; glass; ceramics; and resins suchas polytetrafluoroethylene and lithium terephthalate may be used.

Further, in the above-described embodiments and examples, a terahertzwave resonator with a square patch antenna is used, but the shape of theresonator is not limited to this. For example, a resonator having astructure using a patch conductor having a polygon such as a rectangleand a triangle, a circle, or an ellipse shape may be used.

Further, the number of differential negative resistance elementsintegrated in the semiconductor element is not limited to one, and theresonator may have a plurality of differential negative resistanceelements. The number of lines is not limited to one, and a plurality oflines may be formed. The semiconductor elements described in theabove-described embodiments and examples make it possible to produceoscillating terahertz waves and to detect terahertz waves.

Further, in each of the above-described embodiments, as the RTD, adouble barrier RTD formed of InGaAs/AlAs grown on an InP substrate hasbeen described. However, the semiconductor element according to thepresent invention can be provided not only by such structure andmaterial base but also by combinations of other structures andmaterials. For example, an RTD having a triple barrier quantum wellstructure or an RTD having a quadruple or more multiple barrier quantumwell may be used.

Further, as the material of the RTD, any one of the followingcombinations may be used.

-   -   Materials formed on a GaAs substrate, such as GaAs/AlGaAs,        GaAs/AlAs, and InGaAs/GaAs/AlAs    -   Materials formed on an InP substrate, such as InGaAs/InAlAs,        InGaAs/AlAs, and InGaAs/AlGaAsSb    -   Materials formed on an InAs substrate, such as InAs/AlAsSb and        InAs/AlSb    -   Materials formed on a Si substrate, such as SiGe/SiGe

The above-mentioned structures and materials may be appropriatelyselected according to a desired frequency and the like.

According to the present invention, it is possible to provide efficientgeneration or detection of a terahertz wave in an element having anantenna array structure.

The present invention is not limited to the above embodiments, andvarious changes and modifications can be made without departing from thespirit and scope of the present invention. Therefore, the followingclaims are annexed in order to publicize the scope of the presentinvention.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

1. An element comprising: an antenna array in which a plurality ofantennas are arranged, with each of the antennas including a firstconductor layer, a semiconductor layer that is electrically connected tothe first conductor layer and produces or detects a terahertz wave, asecond conductor layer that is electrically connected to thesemiconductor layer and faces the first conductor layer via thesemiconductor layer, and a dielectric layer that is located between thefirst conductor layer and the second conductor layer; a coupling linethat is connected to the second conductor layer configured to makemutual synchronization between the plurality of antennas at a frequencyof the terahertz wave; and a bias line that connects a power source forsupplying a bias signal to the semiconductor layer and the secondconductor layer, wherein a wiring layer in which the coupling line isformed and a wiring layer in which the bias line is formed are differentlayers, and the bias line is disposed in a layer between the firstconductor layer and the second conductor layer.
 2. The element accordingto claim 1, wherein the coupling line and the bias line cross each otherin plan view.
 3. The element according to claim 1, further comprising athird conductor layer and a fourth conductor layer, wherein the couplingline is formed of the third conductor layer and the first conductorlayer, the bias line is formed of the fourth conductor layer, and thethird conductor layer and the fourth conductor layer are arranged indifferent layers.
 4. The element according to claim 3, wherein the thirdconductor layer and the fourth conductor layer cross each other in planview.
 5. The element according to claim 3, further comprising: a fifthconductor layer disposed in a layer between a set of the third conductorlayer and the fourth conductor layer, and the first conductor layer; anda capacitor structure in which the dielectric layer is interposedbetween the first conductor layer and the fifth conductor layer.
 6. Theelement according to claim 5, wherein the third conductor layer or thefourth conductor layer, and the fifth conductor layer are electricallyconnected via a resistor.
 7. The element according to claim 3, whereinthe first conductor layer, the fourth conductor layer, and the thirdconductor layer are stacked in this order.
 8. An element comprising: anantenna array in which a plurality of antennas are arranged, with eachof the antennas including a first conductor layer, a semiconductor layerthat is electrically connected to the first conductor layer and producesor detects a terahertz wave, a second conductor layer that iselectrically connected to the semiconductor layer and faces the firstconductor layer via the semiconductor layer, and a dielectric layer thatis located between the first conductor layer and the second conductorlayer; a coupling line that is connected to the second conductor layerconfigured to make mutual synchronization between the plurality ofantennas at a frequency of the terahertz wave; and a bias line thatconnects a power source for supplying a bias signal to the semiconductorlayer and the second conductor layer, wherein a wiring layer in whichthe coupling line is formed and a wiring layer in which the bias line isformed are different layers, the element further comprising a thirdconductor layer and a fourth conductor layer, wherein the coupling lineis formed of the third conductor layer and the first conductor layer,the bias line is formed of the fourth conductor layer, the thirdconductor layer and the fourth conductor layer are arranged in differentlayers, and the first conductor layer, the third conductor layer, andthe fourth conductor layer are stacked in this order.
 9. The elementaccording to claim 8, wherein a contact hole that penetrates a part ofthe dielectric layer in a stacking direction is formed, the secondconductor layer and the third conductor layer are connected to eachother by formation of a part of the second conductor layer on a surfaceof the contact hole, and the semiconductor layer that is in a stackingdirection is covered with the dielectric layer.
 10. The elementaccording to claim 1, wherein the bias line is set to have a lowerimpedance than an impedance of the semiconductor layer in a frequencyband lower than the frequency of the terahertz wave.
 11. The elementaccording to claim 1, wherein in the antenna array, the antennas arearranged in a matrix of m×n, where (m≥2, n≥2).
 12. The element accordingto claim 1, wherein the antennas are arranged at a pitch that is anintegral multiple of a wavelength of the terahertz wave.
 13. The elementaccording to claim 1, wherein each of the antennas is a patch antenna.14. The element according to claim 1, wherein the semiconductor layerincludes a negative resistance element.
 15. The element according toclaim 14, wherein the negative resistance element is a resonanttunneling diode.
 16. An element comprising: an antenna array in which aplurality of antennas are arranged, with each of the antennas includinga first conductor layer, a semiconductor layer that is electricallyconnected to the first conductor layer and produces or detects aterahertz wave, a second conductor layer that is electrically connectedto the semiconductor layer and faces the first conductor layer via thesemiconductor layer, and a dielectric layer that is located between thefirst conductor layer and the second conductor layer; a coupling linethat is connected to the second conductor layer configured to makemutual synchronization between the plurality of antennas at a frequencyof the terahertz wave; and a bias line that connects a power source forsupplying a bias signal to the semiconductor layer and the secondconductor layer, wherein a wiring layer in which the coupling line isformed and a wiring layer in which the bias line is formed are differentlayers, and adjacent antennas in the antenna array are connected to acommon bias line disposed between the adjacent antennas.
 17. The elementaccording to claim 16, wherein each of the antennas is connected to thecommon bias line via a lead wire narrower than a width of the antenna.